i used to have a b234r stage 4. i heard of the same thing.
i think the problem is the downpipe goes over the oil pan heating it up causing sludge. then the pickup gets clodged and starves the engine.
thats why i think most new car companies have the intake in the front. with the exhaust in the back.
theres other reasons like engine bay temps and air temps but the downpipe heats it up alot.
i could be wrong tho. just what i think